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Tropical
storms
Lesson Aims

You will be able to explain how and why
hurricanes are formed
Tropical storms are large areas of intense
low pressure and extreme depressions.
On a satellite image, a tropical storm will show
as a huge, swirling mass of cloud.
The eye (calm area, no wind) is at the centre
The track is the path taken by a tropical storm
Formation
These storms occur over water in areas where the sea surface temperature is
at least 27 degrees. (80F)
Essentially, this means that they form between the Tropic of Cancer and the
Tropic of Capricorn (hence the name 'tropical' storm).
They rarely occur within 5 degrees either side of the equator because the
Coriolis force is too weak here.
Formation in detail...
Tropical storms use warm, moist air as fuel. That is why they form only over warm
ocean waters near the equator. The warm, moist air over the ocean rises rapidly
upward from near the surface and becomes saturated with evaporated moisture.
This means that there is less air left at the surface (i.e. low pressure). Air from
surrounding areas with higher air pressure pushes in to the low pressure area to try
to equalise the pressure. Then that “new” air becomes warm and moist and rises,
too. As the warm air continues to rise, the surrounding air swirls in to take its place.
Trade winds cause the moist air to spin inwards. As the warmed, moist air rises and
cools off, the water in the air forms towering cumulonimbus thunderclouds (because
there is a huge amount of condensation). The whole system of clouds and wind spins
and grows, fed by the ocean’s heat and water evaporating from the surface.
natural-disasters-tornadoes.
Measuring tropical storms
The strength of a tropical storm is measured
using the:
Tropical storms usually weaken when they make landfall (i.e. hit the
land) because they are no longer being fed by the energy from the
warm ocean waters. In the northern hemisphere they track (i.e.
move) westwards due to the Coriolis effect
You will see 8 statements –
5 are true reasons as to how a
hurricane is formed.
TASK: In pairs you must decide the 5
which are true.
4 mins…
1. Warm ocean air.
(more than 80 F)
2. Warm and cold air
come together.
3. Damp air has to rise
to make the clouds of
the storm.
4. Winds have to come
together to force air
upward.
5. They start to form
from really bad
thunderstorms.
6. Winds flow outward
above the storm
allowing the air below
to rise.
7. Winds have to be
spinning clockwise.
8. Light winds outside
the hurricane steer
it and let it go.
hurricanes.
So what about the reality for people who
get hit by them??
Watch this and imagine….
Tornado destruction
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